Show usage string for 'git imap-send -h'

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

imap-send.c

@@ -93,6 +93,8 @@ struct msg_data {
 	unsigned int crlf:1;
 };
 
+static const char imap_send_usage[] = "git imap-send < <mbox>";
+
 #define DRV_OK          0
 #define DRV_MSG_BAD     -1
 #define DRV_BOX_BAD     -2
@@ -1491,6 +1493,9 @@ int main(int argc, char **argv)
 
 	git_extract_argv0_path(argv[0]);
 
+	if (argc != 1)
+		usage(imap_send_usage);
+
 	/* init the random number generator */
 	arc4_init();